External pressure vessel framing

ABSTRACT

The present invention relates to a structural form consisting of a hollow member (48) with alternate convex (32) and unstiffened concave (34) surfaces and an internal framing arrangement (36) required to support such a structure such that it is capable of resisting external pressure. This structural configuration reduces the weight and simplifies the construction of such structures in relation to conventional stiffened fixed curvature cylindrical external pressure vessels. Use of such forms is envisaged particularly where external pressure is the dominant loading, for example in the offshore marine and subsea environments.

In the offshore oil business floating structures such assemi-submersibles and tension leg platforms (TLP's) are sometimes usedto support drilling and/or production equipment. These structures arevery weight sensitive in that the less structural steel content used themore equipment payload can be carried. A major structural steel weightcomponent is the outer shell of the columns and pontoons which areusually circular or rectangular in cross section and consist of platecircumferentially or longitudinally stiffened with T stiffeners or bulbflats at close centres. Such a form of construction is both expensiveand time consuming to construct to the fine tolerances required due tothe load capacity sensitivity of compression shells to imperfections.

Both TLP's and semi-submersibles have hull pontoons and columns whichare prismatic elements which may have circular, square, rectangular,hexagonal or other shapes in cross section. In order to resist thehydrostatic pressure around these elements when they are submerged theexternal shell plating is stiffened by transverse stiffeners and/orlongitudinal stiffeners located inside the shell plating to prevent theplate from buckling.

Since this stiffening is very labour intensive to fabricate and addsboth cost and weight to the hull it would be of great benefit toeliminate it altogether. Furthermore, it would be of benefit toconfigure the external plating in such a way that it is not subjected tobending stresses at all and therefore cannot buckle. By so doing, theexternal plating can also be made lighter than it would be as presentlyconfigured in today's floating structures.

It is the purpose of this invention to eliminate the requirement forstiffeners and to lighten the external shell plating on both pontoon andcolumn elements.

In accordance with the present invention there is provided a structuralform comprising a hollow member having a shell with alternate concaveand convex surfaces relative to the longitudinal axis of the member andan internal framing arrangement to support the shell, characterised inthat the concave surfaces are unstiffened and run substantially thewhole of the length of the member, the member being capable of resistingan applied pressure loading.

This structural configuration will reduce the weight and cost of suchstructures when compared with conventional stiffened fixed curvaturecylindrical external pressure vessels. Such structures will offeradvantages over conventional designs particularly where externalpressure is the dominant loading, for example in the offshore marine andsubsea environments.

This framing invention lends itself to any external pressure vessel andis not restricted to TLP's and semi-submersibles or even to waterpressure for that matter. Nor is the idea restricted to steel or anyother metal since this invention would lend itself very well to theintroduction of carbon fibre technology for the exterior shell material.

External pressure vessles are required wherever people or equipment arerequired to be kept dry beneath the sea and this framing invention couldbe used to great advantage in such structures. Examples would includehabitats for people, enclosures for offshore oil production equipment,diving vessels, submarines, buoyancy chambers and tanks, etc. There areinstances where the corner longitudinal framing tubes could be ofadditional use e.g. guides for tension leg platform tethers and pileguides for steel offshore jacket buoyancy legs.

This framing invention would also be of advantage in resisting externalpressure from ice. Whenever a structure becomes frozen in ice greatpressure is exerted by the ice on the structure as the ice expands andthis pressure could be resisted very efficiently by the framinginvention. In addition where the corner pipes are used to form theconvex portions of the shell they could carry steam or other hot gas orfluid to melt the ice local to the structure and thereby reduce theexternal pressure.

Such shapes have not been used before for offshore vessels due to theincreased drag of such a shape in a moving fluid (or when moving througha fluid) but it will be shown that this shape is entirely adequate for astationary TLP or semi-submersible floating production platform. It isalso the case that such a shape is preferable due to its dampingcharacteristics (i.e. the reduction of oscillatory motion) although theextent of this benefit is not determined as yet. It is only relativelyrecently that floating structures were required to stay on station forlong periods of time and were not required to move frequently or rapidlythrough the water.

The invention will now be described by way of example with reference tothe accompanying drawings, in which: FIG. 1 shows a prismatic memberconstructed in accordance with the present invention;

FIG. 2 shows a rectangular structure constructed in accordance with thepresent invention;

FIG. 3 represents a hull for a semi-submersible or TLP consisting ofcolumn and pontoon elements;

FIGS. 4 and 5 show respective cross sections through the column andpontoon of FIG. 3 constructed using known techniques;

FIGS. 6 and 7 show respective cross sections through the column andpontoon of FIG. 3 constructed using the techniques of the presentinvention;

FIG. 8 shows a member with transverse stiffening diaphragms attachedalong its length;

FIGS. 9 and 10 show the spiralling of a prismatic vessel under externalpressure (FIG. 9) and in the absence of an end restraint FIG. 10);

FIG. 11 shows the member of FIG. 8 with a concentric internal element;

FIGS. 12 and 13 show the distortion of a rectangular element under theaction of external pressure;

FIGS. 14 and 15 show a member having internal framing to prevent thedistortion of FIGS. 12 and 13;

FIG. 16 shows an internal framework for prevention of distortion withoutuse of a concentric internal element;

FIGS. 17 to 20 illustrate the positioning and construction oflongitudinal bulkheads within the member; FIGS. 21 to 26 illustrate thepositioning and construction of stiffened (FIGS. 21 to 24) andcorrugated (FIGS. 25, 26) transverse bulkheads within the member;

FIGS. 27 and 28 show the construction of the outer skin of the member;

FIG. 29 represents a member suitable for use as the pontoon of FIG. 3;

FIG. 30 represents a member suitable for use as the column of FIG. 3;and

FIGS. 31 to 33 illustrate the transition from concave membrane to flatplate at the extremity of a member.

FIG. 1 shows a hollow structural member constructed in accordance withthe present invention. It has a number of longitudinal corner tubes 30with convex corner plating 32 attached thereto. Intermediate eachadjacent pair of corner tubes 30 is a section of unstiffened concavespan plating 34. Within the member is an internal framework 36 whichsupports the corner tubes.

The framing invention can also be used for structures which must providerelatively large dry areas in a subsea environment. An example is shownin FIG. 2 wherein a large number of parallel pipes 30 form an almostflat roof with concave membrane shell plate 34 between the pipes.Similar framing is used for the floor of the structure. Internal tubularframing 36 supports the pipes 30 which may also be used as serviceducting. A typical hull for a semi-submersible or TLP is shown in FIG.3. It might consist of any number of pontoon 38 or column 40 elementsbut for simplicity a square closed ring pontoon 38 of four elements withfour vertical columns 40 is shown. Cross sections of the column (AA) andpontoon (BB) constructed using known techniques and using T-stiffeners42 and ring stiffeners 44 are shown in FIGS. 4 and 5 respectively. FIGS.6 and 7 respectively show the same cross sections of the columns andpontoons using the present invention and showing the simplicity ofconstruction.

To further illustrate the framing technique only a rectangular sectionis shown in FIG. 8, however, other sections would be framed in a similarmanner.

It is anticipated that diaphragms 46 and/or transverse bulkheads wouldbe set up and jigged into the correct relative locations while thecorner pipes 30 with cover plates 32 already in position were attached.After two such pipes 30 were attached to the top corners of thediaphragms 46, the concave top shell plating 34 between them would belowered onto the corner pipes 30 and welded along its edges to the coverplates 32. Full penetration welds would join the shell plate to thecover plates using the corner pipes 30 as back-up and the welding wouldbe done in the downhand position. The entire element would be rotated toattach the other shell plates in a similar manner using downhandwelding.

A prismatic external pressure vessel element 48 having both concave 34and convex 32 surfaces as shown in FIG. 9 could have low torsionalstiffness in some cases. It can be expected that in the absence of fixedend restraint such a vessel would undergo a decrease in volume underexternal pressure by spiralling, i.e. a concave "flute" of the elementwhich is originally straight would spiral under increased pressure asshown in FIG. 10.

The end fixity found when such an element 48 is incorporated into such astructure as a TLP or semi-submersible will tend to resist suchdistortion but additional resistance can be obtained by utilizing thehigh torsional resistance of a smaller prismatic element 50 withstraight (unfluted) sides coaxial with the larger fluted element asshown in FIG. 11.

It should be noted that such an internal tube is often beneficial inmarine vessels since it provides a reserve of buoyancy in the event ofwater ingress through the outer shell. It is claimed that such a flutedelement has not been used in the past precisely because it will tend tospiral under external pressure. In this case the invention is also basedon the claim that an inner unfluted tube may be used in conjunction withthe external fluted shell as one method of preventing this action.

Considering the case of a rectangle, i.e. a prismatic member with across section that is "almost" a rectangle but with concave sides, underthe action of the uniform external pressure (P), this element will tendto collapse as shown in FIGS. 12 and 13.

The internal framing must resist this collapse mechanism. Manyalternatives are possible, one being shown in FIG. 14 and illustratingthe framing (diaphragms) in the form of compression posts 52 and tensionstruts 54 that could be used between transverse bulkheads. The diaphragmdesign above would be a minimum requirement to prevent the form ofcollapse described. An alternative design would add compression posts 56along the short dimension (a) as shown in FIG. 15.

The transverse diaphragm framing proposed above would not be used if itwere not less expensive than using a plated bulkhead, i.e. simplyspacing bulkheads closer together.

An inner tube 50 at the axis of the element is not the only method ofpreventing collapse by spiralling due to insufficient torsionalstiffness. Other methods exist for increasing the torsional stiffness ofthe element sufficiently to prevent such collapse which do not requirethis central tube and an example is shown for an element having atriangular cross section in FIG. 16.

In this example the torsional stiffness is increased by framing 58between adjacent pipe corners 30 to make trusses. Diagonal members 60 inthese trusses will increase the torsional stiffness of the elementsufficiently to prevent spiral collapse. An alternative would be toreplace the diagonal members 60 with light plates acting in shear. Ifthe diagonals 60 were left out entirely, some resistance would still beprovided because the truss acts as a Vierendeel girder.

Elements having any number of sides could be framed in a similar mannerand in each case the internal framing would have to be such that itprovided the required torsional stiffness between transverse bulkheads.Framing such as this might be preferred in those sub-sea structureswhere a central tube is not of advantage in providing reserve bouyancyor it obstructs the passage of people or location of equipment. As anexample, portions of the pontoons of a TLP might require a central tubebut other portions, such as areas for pump rooms may not. In thisexample the pump rooms could be framed by forming trusses betweenadjacent corner pipes as was shown above in FIG. 16.

If the external pressure vessel is part of a floating vessel such as aTLP or semi-submersible then internal bulkheads will be required alongthe axis of the element as well as transverse to the axis. This divisionis to limit the volume which is flooded in the event of water ingressthrough the outer shell and to maintain upright stability. The mostdesirable framing plan for longitudinal bulkheads 62 would be to placethem between the central tube 50 (probably a cylinder in most cases) andthe corner pipe beams 30 as illustrated in FIGS. 17 and 18.

The longitudinal bulkheads 62 are best formed from corrugated platepanels and are most ideally located as shown in FIGS. 19 and 20 tosupport the high compressive forces necessary to prevent the corner pipebeams 30 from bending towards the centre of the element under the actionof external pressure. Obviously such bulkheads will have to becorrugated or stiffened to prevent buckling and their use must be keptto a minimum to prevent unnecessary additional weight and expense.

Since longitudinal bulkheads 62 will remove the requirement to designthe pipes they support for bending loads they are preferable totransverse bulkheads 64, but they will not limit the need for transversebulkheads 64 altogether.

Transverse bulkheads 64 must provide a watertight seal where they meetthe outer membrane shell 34 but must not provide rigid support for theshell as this would prevent the shell from acting as a simple membranewith single curvature. It is only where a transverse bulkhead 64 meetsthe outer membrane 34 that there is the potential structural problems.This problem must be overcome by some sort of "sort support". If,however, a transverse bulkhead meets a longitudinal bulkhead or an innertube, rigid support made by welding a rigid bulkhead to a rigid elementis entirely satisfactory.

One method for allowing "soft support" at the outer membrane shell isillustrated in FIGS. 21 to 24.

As water pressure (P) acts upon the outer membrane shell 34 it will tendto compress the elastomeric seal 66 down against the edge of thetransverse bulkhead 64. As water pressure (P) acts against one side ofthe transverse bulkhead 64 the elastomeric seal 66 will compress on theopposite side thus helping the sealing action.

Other structural details which would provide the required in-planediaphragm flexibility include the use of a corrugated plate where thecorrugations are normal to the resultant external force on a membranepanel as shown in FIGS. 25 to 26. For such a construction elastomericseals may simply be replaced by a weld joint 70 between the corrugatedplate 68 and the outer membrane shell 34.

A preferred detail is to have pipe beams 30 to help form the convex"corners" of the section and to carry the load from the concave sections34. The design must also consider the case where an adjacent convexpanel has ruptured due to ship impact or other cause thereby alteringthe normal loading on the pipe beam. A preferred detail is shown in FIG.27.

A feature of the invention is that welds joining the convex corner shell(cover) plates 32 to the concave side shell plates 34 can be madeutilising the pipe beam 30 as a backup. This will provide for simplefabrication as well as good structural design. It should be emphasised,however, that these single sided welds 72 made with a backup cannot beexpected to be defect free and inspection of the reverse side will beimpossible. For this reason welding imperfections must be allowed for inthe design. In areas where inspection of the back of the weld is deemednecessary a built up area of weld 74, called a nib, can be provided bywelding on the corner pipe 30 a raised portion of weld and then grindingit to provide a suitable weld preparation 76 as shown in FIG. 28. Theouter membrane shell 34 is then brought up to the nib 74 and held inplace by a temporary weld support 78 until the weld 80 is completed.Removal of the weld support 78 permits inspection of the rear 82 of thecompleted weld 80.

For a given pressure head and thickness of external shell plate theremust be a certain minimum sag to span ratio in the shell in order tolimit the membrane stress in the shell plate. It has been establishedthat a minimum sag to span ratio of about 10% is necessary.

There exist an infinite variety of possibilities for using rectangles,octagons, and other shapes in various combinations for the hull pontoonsand columns of TLP's or semisubmersibles. Some designs are better thanothers for particular applications. The general statement can be madethat the more sides to the cross section the thinner the membrane shellplate can be made. This must be balanced, however, with the additionalweight and cost of the corner pipe members and other internal framing.

The option shown below in FIG. 29 for a pontoon 38 illustrates asuitable compromise.

The choice of this shape is largely governed by the choice of on octagonshape for the columns 40 to which the pontoons will be framing. Anoctagon shape is a suitable choice for a column as shown in FIG. 30.

It can be seen that the corner pipe 30 can be placed such the pipes inthe hull pontoons 38 frame directly into pipes from the columns 40.These pipe connections will be points of high stress concentration andwill be designed as are the pipe nodes familiar in offshore platform(jacket) fabrication.

A proposed detail where the shell plating of the columns meets the shellplating of the pontoons is to gradually change from sagging membrane 34to flat plate 84 so that the flat plate of the column joins to the flatplate of the pontoon. This is done by a transitional plate 86 in theform of a membrane shell having decreasing curvature along its length.Such transitional arrangements will be provided on the stub ends forboth the pontoons and columns at the corner nodes. A suitablearrangement of this detail is shown in FIGS. 31 to 33. The detail shownin FIGS. 32 and 33 can also be used in subsea applications where it isnecessary to close the ends of the structure. Transitioning to flatplate at the end of the element will allow welding at the corners tothis flat plate.

Although the examples shown have been described with reference toprismatic elements having corner pipes which are straight and parallel,this does not preclude application of the invention to structures suchas submarines which may require curved and/or non-parallel corner pipes.Whilst the majority of the examples described refer to floating vesselsi.e. TLP's and semisubmersibles, it will be understood that there aremany other areas where the framing invention described herein wouldprovide advantages over current tecnniques.

We claim:
 1. A structural form comprising a hollow member having a shellwith alternate concave and convex surfaces relative to the longitudinalaxis of the member and an internal frame arrangement to support theshell, wherein the concave surfaces are unstiffened and runsubstantially the whole of the length of the member, the member beingcapable of resisting an applied external pressure loading, and saidalternate concave and convex surfaces adjoining each other at aplurality of spaced locations on said shell and defining an outerperipheral surface of said shell, said outer peripheral surface having asmoothly curved configuration which is generally free of curvaturediscontinuities.
 2. A member according to claim 1, characterized in thatit is a prismatic element having longitudinally extending frame membersdisposed intermediate adjacent concave surfaces.
 3. A member accordingto claim 2, characterized in that said shell includes alternate concaveand convex shell portions which are joined together and are joined tothe longitudinally extending frame members.
 4. A member according toclaim 1, characterized in that it is a prismatic element havingstraight, parallel elongate corner frame members which support saidconvex shell surfaces, and said shell having concave shell portionsattached between respective pairs of said corner frame members.
 5. Amember according to claim 1, characterized in that the concave surfacesof said shell have a curvature only in a direction transverse to thelongitudinal axis of the member.
 6. A member according to claim 1,characterized in that the internal frame arrangement includes anelongate longitudinal stiffening member.
 7. A member according to claim1, characterized in that the internal frame arrangement includes atransverse bulkhead.
 8. A member according to claim 1, characterized inthat the internal frame arrangement includes a longitudinal bulkhead. 9.An elongate hollow structural member of generally polygonalcross-section having an internal support frame work and an outer shellcomprising a plurality of adjacent elongate shell portions, whereinalternate shell portions of said plurality have a concave and convexsurface respectively relative to the longitudinal axis of the member,each concave and convex surface running substantially the whole of thelength of the member, and said alternate concave and convex surfacesadjoining each other at a plurality of spaced locations on said shelland defining an outer peripheral surface of said shell, said outerperipheral surface having a smoothly curved configuration which isgenerally free of curvature discontinuities.
 10. A member according toclaim 9, characterised in that it is a prismatic element havinglongitudinally extending frame members disposed intermediate adjacentconcave surfaces.
 11. A member according to claim 10, characterised inthat said alternate concave and convex shell portions are joinedtogether and are joined to the longitudinally extending frame members.12. A member according to claim 9, characterised in that it is aprismatic element having straight, parallel elongate corner framemembers which support said convex shell surfaces, and having saidconcave shell portions attached between respective pairs of said cornerframe members.
 13. A member according to claim 9, characterised in thatthe concave shell portions have a curvature only in a directiontransverse to the longitudinal axis of the member.
 14. A memberaccording to claim 9, characterised in that the internal support framework includes an elongate longitudinal stiffening member.
 15. A memberaccording to claim 9, characterised in that the internal support framework includes a transverse bulkhead.
 16. A member according to claim 9,characterised in that the internal support frame work includes alongitudinal bulkhead.
 17. An elongate hollow structural member ofgenerally polygonal cross-section having an internal support frame workand an outer shell comprising a plurality of adjacent elongate shellportions, wherein alternate shell portions of said plurality have aconcave and convex surface respectively relative to the longitudinalaxis of the member, each concave and convex surface runningsubstantially the whole of the length of the member, said alternateconcave and convex surfaces adjoining each other at a plurality ofspaced locations on said shell and defining an outer peripheral surfaceof said shell, said outer peripheral surface having a smoothly curvedconfiguration which is generally free of curvature discontinuities, saidmember being an external pressure vessel for forming one of a pontoonelement and a column element of a floating structure.
 18. Asemisubmersible floating structure, comprising:one of a hull pontoon anda deck support column which includes a hollow member having a shell withalternate concave and convex surfaces relative to the longitudinal axisof the member and an internal frame arrangement to support the shell,said alternate concave and convex surfaces adjoining each other at aplurality of spaced locations on said shell and defining an outerperipheral surface of said shell, said outer peripheral surface having asmoothly curved configuration which is generally free of curvaturediscontinuities, wherein the concave surfaces are unstiffened and runsubstantially the whole of the length of the member, the member beingcapable of resisting an applied external pressure loading.
 19. A methodfor producing a structural form which is a hollow member capable ofresisting applied external pressure loading, which method comprisesproviding a shell with alternate concave and convex surfaces relative tothe axis of the member, said alternate concave and convex surfacesadjoining each other at a plurality of spaced locations on said shelland defining an outer peripheral surface of said shell, said outerperipheral surface having a smoothly curved configuration which isgenerally free of curvature discontinuities, and providing an internalframing arrangement to support the shell, characterised by the use ofunstiffened concave surfaces along substantially the whole of the lengthof the member.
 20. A structural form comprising a hollow member having ashell with alternate concave and convex surfaces relative to thelongitudinal axis of the member and an internal frame arrangement tosupport the shell, said alternate concave and convex surfaces adjoiningeach other at a plurality of spaced locations on said shell and definingan outer peripheral surface of said shell, said outer peripheral surfacehaving a smoothly curved configuration which is generally free ofcurvature discontinuities, wherein the concave surfaces are unstiffenedand run substantially the whole of the length of the member, the memberbeing capable of resisting an applied external pressure loading, saidmember being an external pressure vessel for forming one of a pontoonelement and a column element of a floating structure.
 21. Asemisubmersible floating structure, comprising:one of a hull pontoon anda deck support column which includes an elongate hollow structuralmember of generally polygonal cross-section having an internal supportframe work and an outer shell comprising a plurality of adjacentelongate shell portions, wherein alternate shell portions of saidplurality have a concave and convex surface respectively relative to thelongitudinal axis of the member, said alternate concave and convexsurfaces adjoining each other at a plurality of spaced locations on saidshell and defining an outer peripheral surface of said shell, said outerperipheral surface having a smoothly curved configuration which isgenerally free of curvature discontinuities, each concave and convexsurface running substantially the whole of the length of the member.